PLATAXISTA. 515 



runs close to the canal of the nerve. These canals can only be traced by fracturing 

 the bone across at various intervals, and thus tracing them to then" destinations. 

 Into the deep notch in front of the cochlea is received the strong process of the 

 tympanic before the malleus, and which is usually fractured off from the tympanic 

 and left attached to the periotic, so firmly is it embraced by it. A round ossicle 

 frequently intervenes between the anterior end of this section of the periotic and 

 the petro-pterygo-tympanic process and the pterygoid, and appears to be a separated 

 portion of the hook-like process. 



The posterior division of the periotic forms by far the larger section and is 

 thick and massive, being defined anteriorly, on the under surface, by the afore-men- 

 tioned notch and superiorly by the anterior border of the cochlea. Viewed from 

 the upper surface, it is oblong, placed obKquely from without inwards and forwards 

 on the under sm-face of the skull, its inner rounded extremity being the free 

 border of the cochlea which projects into the hinder portion of the great chamber 

 at the base of the skuU. The meatus auditorius internus looks inwards and upwards 

 with a sHghtly forward direction, and its external upper margin is thin and spicular, 

 projecting considerably above the interval, wliich is also a thin raised ridge. The 

 margin of the orifice as a whole gives rise to a slight tube, which points towards 

 the opening at the base of the skull, through which the seventh pair of nerves pass 

 out, and which corresponds to the deep furrow on the basi-occipital behind the 

 powerful posterior wing of the basisphenoid ; the periotic in the adult being entirely 

 outside the cranial waU, while in yoimg skulls the cochlear portion projects 

 strongly into the cavity of the skull. The meatus auditorius internus is a deep 

 circular pit, marked by a series of cribriform spirals with the opening of the aque- 

 duct of EaUopius near the bottom of the pit, in its outer wall. Immediately external 

 to the meatus auditorius internus and nearly in a line with its hinder margin, 

 there is a prominent well-defined pit, in the bottom of which occurs the minute 

 aqueduct of the vestibule, the anterior wall of which usually bears a long spicule. 

 It may be taken as a guide to the position of the semi-circular canals which occur in 

 the petrous substance of the bone immediately anterior to it ; the aqueduct of the 

 vestibule opening internally, anterior to the inferior end of the superior or posterior 

 semi-circular canal. Below this, on the posterior border of the bone, is the verti- 

 cally, somewhat oval, and large fenestra rotunda, in the bottom of which are visible 

 the two spiral laminse of the modiolus and wall of the cochlea separated by a narrow 

 intervening fissau'e. Between the fenestra rotunda and the internal auditory meatus 

 is the short canal, the aqueduct of the cochlea. The under surface of this portion 

 of the posterior division of the periotic presents two powerful, fang-hke processes, 

 the anterior of which fits into the deep pit on the postauditory process of the squa- 

 mosal, the posterior being embraced by the exoccipital and the vaginal process of 

 the tympanic. The deep notch between these two processes is completely occupied 

 by the inner and posterior walls of the postauditory pit, which so grasp and embrace 

 the anterior process that the periotic can only be removed by f ractm-mg the walls 

 of the pit. 



